Brazing is widely used as a method for jointing aluminum products including a number of fine jointing portions, such as aluminum heat exchangers and mechanical components. Brazing aluminum requires breaking an oxide film covering the surface, and bring the molten brazing material into contact with base metal or brazing material molten in the same manner. Methods for breaking an oxide film are broadly classified into a method of using a flux and a method of heating the oxide film in vacuum, and both of them are put to practical use.
An application range of brazing is wide, and the most typical example is a heat exchanger for vehicles. Most of heat exchangers for vehicles, such as radiators, heaters, condensers, and evaporators, are made of aluminum, and most of them are manufactured by brazing. Among brazing methods, a method of applying a noncorrosive flux and heating it in a nitrogen gas atmosphere occupies the majority part of the methods at present.
In recent years, due to change in the driving system in electric cars and hybrid cars, heat exchangers equipped with electronic components, such as an inverter cooler, appear, and residue of a flux is regarded as problem in increasing cases. For this reason, some of inverter coolers are manufactured by vacuum brazing in which no flux is used. However, vacuum brazing requires high equipment cost and high maintenance cost for the heating furnace, and has problem in productivity and brazing stability. Such a situation increases needs for jointing without using a flux in a nitrogen gas furnace.
To respond to the needs, various methods have been presented as a method of enabling brazing, without using a flux, in an inert gas atmosphere, by diffusing Mg into a brazing material during brazing heating. Examples of the methods include a method of diffusing Mg added to the core material into the brazing material, and a method of diffusing Mg added to the sacrificial anode material disposed between the core material and the brazing material into the brazing material. These methods prevent formation of an oxide film on the surface of the brazing material in manufacturing of a clad material or during brazing heating, and Mg effectively acts on destruction of the oxide film on the surface of the brazing film.
However, destruction of the oxide film on the surface of the brazing material is not sufficiently performed, by merely diffusing Mg into the brazing material. Without application of a flux, sufficient fillet formation ability cannot be exhibited in a practical joint having a clearance. By contrast, another method has been presented. In the method, Bi is added to the brazing material, to promote the action of Mg of breaking the oxide film, and greatly improve the brazing properly without applying a flux. However, when Bi exceeding 0.05% is added to the brazing material, a Bi-based oxide is generated on the surface of the brazing material at the stage of manufacturing of the material. Brazing the material with the Bi-based oxide without any treatment causes discoloration, and the problem of marked deterioration in brazing properties.
The mechanism of generation of a Bi-based oxide can be explained from the fact that Bi has a low melting point (approximately 270° C.) and is hardly dissolved in aluminum. Specifically, in hot rolling and/or annealing, Bi scattered in a state of a substantially pure substance is molten, to adsorb oxygen and form a thick Bi-based oxide film, and deteriorates the brazing properties. Reducing the Bi quantity in the brazing material can be one way to suppress it, but causes failure in full achievement of the effect of Bi. Although a certain effect is obtained by performing pretreatment before brazing to remove the Bi-based oxide, reoxidation occurs during preheating for brazing in an inert gas atmosphere with oxygen concentration of 20 ppm or higher, and the effect of the pretreatment is lost. By contrast, excellent brazing properties are exhibited in a low-oxygen atmosphere. However, achieving a low-oxygen atmosphere requires much cost, and is not practical.